This invention relates generally to a water feed system for boilers and the like and specifically, it relates to improved apparatus for providing liquid level control.
Control of liquid level in tanks, boilers, stills and the like, has largely been accomplished by float devices which, as the liquid level rises, operate to close a feed valve, or to throw a switch to cut off a feed pump. In the case of boilers and stills, it is a further requirement of any control system that it function to cut off the heat source (burners, electric heating elements, or the like) if the liquid level falls so low as to endanger the contained vessel or the heating source. In the case of boilers and stills under pressure or vacuum, these float systems also require that the float position be transmitted to the outside switch or feed valve through a packing gland or bellows, magnetic follower, or the like. With such arrangements, considerable trouble has resulted by reason of wear and variable friction and float hangups.
In recent years, such problems have been avoided by hanging electrically conducting probes down from above the liquid line into the liquid. Normally, three probes are used with the tip of one probe being placed at the highest desired water level so that when water contacts this probe a circuit is closed through a relay to shut off the feed system. A second probe has its tip positioned somewhat below the tip of the first probe, and is wired through another relay so that when the water level breaks from this probe, the feed pump or solenoid is turned on. The tip of the third probe extends lower than the tip of the second probe, and is wired so that when the water level breaks below this probe, the heat source (burner, heating elements and the like) is cut off so that no damage occurs. This latter cut off can automatically bring the heat source back on when water again contacts this lower probe, or a drop out can be inserted so that manual resetting is required. These systems require in addition to the three probes at least two special relays.
In view of the cost of these special relays and the three probes (with their fittings to get in to the boiler) some small boilers operating near atmospheric pressure have used a single probe together with a timer. With an arrangement such as this, when the water level breaks from the probe, it must stay continuously broken from the probe for a predetermined number of seconds, for example, approximately five seconds, after which the feed pump or valve is opened and feeding continues until the water level again rises to the probe. The purpose of the delay is to prevent over working the control system by reason of waves and splashing breaking contact with the probe even though the water is at a satisfactory level, and to allow the water level to drop appreciably before feeding. With single probe systems of this type only a single relay is required. For protection against overheating in the event that the water does not cover the boiler tubes, electrical heating elements and the like, other means such as temperature cut offs are used. The latter usually are much less costly than a second probe and relay. However, these temperature cut offs are much slower to act than a low water cut off probe and, in most cases, the fast reaction time of a probe is highly desirable.
Accordingly, it is an object of the present invention to provide a simple single probe, single relay control system that will stop the feed at maximum level, turn the feed on at a level below that, and quickly shut down the heat source at low water to prevent damage to the heated portions of the boiler.
An advantage of the control system of the present invention lies in the fact that a low water cut off is provided, but only a single probe and single relay are required.
Generally, the single probe, single relay control system of the invention is connected and arranged to:
1. Turn off the water feed when the water level reaches the probe;
2. Turn on the water feed after the water level has broken below the probe for a predetermined period, typically, five seconds;
3. If the water rapidly rises to the probe after having broken from the probe, the feed is turned off, ending the normal cycle; and
4. If the water, after having broken from the probe does not rise again to the probe in a second predetermined time period, typically ten seconds, the heat source is shut down. The heat source can be locked out or an alarm energized, if such a condition occurs.